1. Field of the Invention
This invention relates to the removal of impurities from a gas or a liquid. More particularly, the invention is concerned with an improved process for the regeneration of an amine which has been utilized as an absorbent in the removal of impurities such as H.sub.2 S and/or CO.sub.2 from a gas or liquid.
2. Description of the Prior Art
Various operations for the recovery of substantially pure gaseous or liquid streams by chemical treating or selective solvent treatment of an impure gaseous or liquid stream are known. U.S. Pat. No. 2,860,030 to Walter A. Goldtrot, Nov. 11, 1958 describes such processes or methods using amines.
In the known absorption operations, the enriched absorbent or solvent, after being withdrawn from the absorber column, is passed into a regenerator column where the absorbed H.sub.2 S and/or CO.sub.2 are stripped by countercurrent contacting with steam. The steam is generated at the bottom of the regenerator column by boiling the solution in an indirectly heated tubular heat exchanger or "reboiler". The heat for this boiling may come from steam, or from any hot fluid, or from direct firing. The countercurrent contacting with steam in the regenerator column, followed by boiling, will strip the H.sub.2 S and or CO.sub.2 from the solution down to a very low residual level. The hot stripped solution, known as "lean solution" is withdrawn from the reboiler, cooled, and sent back to the absorber column to complete the cycle of absorption and regeneration. Cooling to a temperature of 120.degree. F. or lower is essential when using alkanolamines since their capacity for absorbing acid gases decreases with increasing temperature. The alkanolamine, which has a boiling point substantially higher than that of water, will remain almost completely in the liquid phase, with only traces appearing in the vapor.
As the steam generated in the reboiler passes up the regenerator column, a portion of it condenses, giving up its latent heat of vaporization to the absorbent liquid. The amount of heat given up will be equal to the sum of the amount required to heat the absorbent liquid from the temperature at which it enters the regenerator column to the boiling temperature in the reboiler plus the amount required to supply the chemical heat of reaction to decompose the amine/acid gas compounds, releasing the acid gas as a vapor.
The remaining steam will pass overhead from the regenerator column, mixed with the acid gas stripped from the absorbent liquid. The minimum amount of steam that can be present in this mixture is the amount required to saturate it at the temperature of the incoming spent absorbent stream. The higher this temperature, the greater the proportion of steam to acid gas required to give a saturated vapor. A saturated vapor is one in which the partial pressure of water vapor is equal to the vapor pressure of water above the solution at the same temperature. The rising steam vapors cannot be cooled by the absorbent liquid to a temperature lower than that of the incoming spent absorbent stream, therefore the higher this temperature, the greater the amount of steam that must leave the regenerator column with the acid gas.
In current widely accepted design and operating procedures, it is generally held that to obtain an adequate degree of stripping in the regenerator column, a substantial excess of steam in the overhead over the saturation amount is required. For example, in Gas Purification, 2nd Edition, by Fred C. Riesenfeld and Arthur L. Kohl, published by Gulf Publishing Co., Copyright 1960 through 1974, the authors specify that the volume ratio of steam to acid gas in the regenerator column overhead, known as the reflux ratio, should be in the range of 2:1 to 3:1 to obtain adequate stripping of monoethanolamine solution. This compares with the minimum, saturation reflux ratio for a column operating with a top pressure of 29.7 psia as follows:
______________________________________ Temperature of Reflux Ratio Solution Entering Regenerator At Saturation ______________________________________ 190.degree. F. 0.425 200 0.515 210 0.770 220 1.110 ______________________________________
If the steam generated in the reboiler is in excess of the amount required for heating the absorbent liquid from the temperature at which it enters the regenerator column to the reboiler temperature, plus the heat of reaction to vaporize the absorbed acid gases, plus the saturation steam at the temperature of the entering spent rich absorbent liquid, then the temperature of the vapor rising in the regenerator column will not be reduced to the temperature of the entering absorbent liquid, but it will remain at a higher level, equivalent to its dew point temperature with respect to the absorbent liquid. For a 20 percent monoethanolamine solution, at 29.7 psia, if the reboiler steam is increased to give a reflux ratio of 2.0, the top vapor temperature will be 232.degree. F., while at 3.0, it will be 239.degree. F., even though the temperature of the rich solution entering the regenerator may be lower than these values.
Since the cost of heat for regenerating the solution typically represents as much as two-thirds of the variable operating cost for an amine treating system, there is an economic incentive to reduce this heat to the minimum. However, it cannot be reduced to a value less than the total required to satisfy the three requirements enumerated; to heat the spent solution to the temperature of the reboiler, to supply the heat of reaction for releasing the acid gases from solution, and to supply at least the amount needed to saturate the overhead vapor at the temperature of the entering solution.
Since the hot lean solution leaving the reboiler at a temperature in the range of 230.degree. to 280.degree. F. must be cooled below 120.degree. F., it is a further widely accepted aspect of current design to do a portion of this cooling by passing the hot lead stream through a heat exchanger where it gives up heat to the entire spent rich absorbent stream coming from the bottom of the absorber. Any heat absorbed by the rich absorbent stream in this exchanger increases the temperature at which it enters the regenerator column, reducing the amount of steam required to be condensed in the column to bring the absorbent stream up to the temperature of the reboiler.
Several attempts have been made to reduce the overall cost associated with the regeneration of absorbent liquid streams. In U.S. Pat. No. 3,690,861 the patentee sought to reduce capital investment costs by devising a system in which the spent absorbent/lean absorbent heat exchanger, the overhead cooler-condenser, the reflux drum, and the reflux pump, ordinarily constructed and used with a conventional stripper or regenerator, could be eliminated. This was accomplished by passing the entire spent absorbent liquid directly to the top of the regenerator column without prior essential heating by a heat exchanger. In the process as outlined heat exchange by gas liquid contact was substituted for the heat exchange normally practiced in the usual spent absorbent/lean absorbent exchanger, thereby eliminating the cost of the heat exchanger and associated equipment.
While capital investment was considerably reduced in the process of U.S. Pat. No. 3,690,816 no consideration was given to how the elimination of heat exchangers would affect overall heat requirements of the system. In fact, it has been found that elimination of heat exchangers increases the overall heat requirements of the system.
Thus, while initial capital expenditures are considerably lessened, long term operating expenses, especially in view of rising energy costs, would be higher in the process of U.S. Pat. No. 3,690,816.
In accordance with the present invention it has been found that contrary to prior art processes in which the entire spent absorbent stream was preheated to optimize energy requirements, a substantial reduction in overall energy requirements can be realized by heating only a portion of the impurity-rich absorbent stream and passing it to an intermediate point of the regenerator column and passing a portion of said spent absorbent stream without prior heating directly to the top of the regenerator column. It has furthermore been found that the proportions of the split between preheated and unheated streams, and the quantity of heating medium supplied to the reboiler can both be controlled near their optimum values by measuring and controlling two temperature differences by use of conventional temperature measuring devices.